I have an external disk connected to MacBook via USB. From time to time the system alerts that the disk is unreadable, but sometimes it seems to connect and I can browse it in Finder, and even open some of its files with Quick Look, but some folders became empty although the disk has the same amount of free space. The Disk Utility can't repair the disk with alert: couldn’t unmount volume for repair. How can I possibly restore my data? My computer is MacBook Pro (early 2011), OSX Yosemite (10.10). The disk has its own power adapter. I've tried Data Recovery app, but it doesn't see the disk file system and can't scan it, while at the same time I can open some files on the disk in Finder... I also need to force eject the disk because it never ejects normally.
I've tested the disk with the case and power adapter which work for sure (with another HD).